The present invention relates to a method and apparatus for measuring the spacing between two separated elements, as for example, for indicating a gap width between the grinding discs of a grinding apparatus for preventing metallic contact between the discs.
It is known previously for measuring of the spacing between two discs of magnetic material to measure the reluctance (the magnetic resistance) in an electric circuit by means of an inductive position indicator disposed in one of the discs, in the air gap between the indicator and the opposite disc forming part of the circuit. The inductance or the coupling intensity of one or two coils is measured by supplying and measuring an alternating current. However, this method has the drawback that great losses of iron are caused both in the indicator and the opposite disc. The iron losses result in great dependence on temperature and deteriorated magnetic conductivity of the iron. In order to eliminate the iron losses and the solving problem of dependence on temperature the measuring of the reluctance can be performed by means of DC fields. However, this involves the drawbacks that the indicator becomes susceptible to external disturbing fields, and in addition the deflection of the indicator becomes dependent on the remanence inherent to the indicator and the disc material. The method involves also that great demands on the DC field meter regarding stability when an absolute value of the field intensity shall be measured.